<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .b1{
            margin: 10px;
            background-color: white;
            outline: white;
            border: solid 1px white;
        }
        .b1:focus{
            background-color: red;
            color: white;
        }
        div>label{
            margin: 10px;
            color: darkgrey;
            font-size: 14px;
        }
    </style>
</head>
<body>
    <div>
        <label for="">难度:</label>
        <input type="button" value="全部" class="b1">
        <input type="button" value="初级" class="b1">
        <input type="button" value="中级" class="b1">
        <input type="button" value="高级" class="b1">
    </div>
</body>
</html>